A positive temperature coefficient (PTC) element exhibits a PTC effect that renders the same to be useful as a circuit protecting device, such as a resettable fuse. The PTC element includes a PTC polymer material and first and second electrodes attached to two opposite surfaces of the PTC polymer material.
The PTC polymer material includes a polymer matrix that contains a crystalline region and a non-crystalline region, and a particulate conductive filler dispersed in the non-crystalline region of the polymer matrix and formed into a continuous conductive path for electrical conduction between the first and second electrodes. The PTC effect is a phenomena that when the temperature of the polymer matrix is raised to its melting point, crystals in the crystalline region start melting, which results in generation of a new non-crystalline region. As the new non-crystalline region is increased to an extent to merge into the original non-crystalline region, the conductive path of the particulate conductive filler will become discontinuous and the resistance of the PTC polymer material will be sharply increased, thereby resulting in an electrical disconnection between the first and second electrodes.
FIGS. 1 to 4 illustrate consecutive steps of a conventional method of making a PTC circuit protection chip device. The conventional method includes: preparing a PTC composition containing a polymer material and a conductive filler; compounding the PTC composition under a temperature of about 200° C. and extruding the PTC composition to form PTC pellets 101; hot pressing the PTC pellets 101 in a mold 100 under about 200° C. to form a compounded sheet 11 (see FIGS. 1 and 2); disposing the compounded sheet 11 between two metal foil sheets 12 to form a stack 10 (see FIG. 2); hot pressing the stack 10 under a temperature of about 200° C. (see FIG. 2); cutting the hot-pressed stack 10 into a plurality of chips 13, each of which includes first and second electrodes 131 and a PTC body 132 sandwiched between the first and second electrodes 131 (see FIG. 3); cross-linking the PTC body 132 of each chip 13 by irradiating it using Co_60 gamma ray; and welding first and second terminal leads 14 respectively to the first and second electrodes 131 with a solder material through welding techniques (see FIG. 4). The welding temperature depends on the solder material employed, and is normally about or greater than 260° C. Since the aforesaid welding operation is required to be operated under a temperature of about or greater than 260′C, the high welding temperature unavoidably results in degradation of the PTC body 132 of the chip 13, which, in turn, degrades electrical properties and the PTC effect of the chip 13 and reduces the service life of the chip 13.